Combined flow, pressure and temperature sensor

ABSTRACT

The invention relates to a device for measuring pressure, temperature and/or flow velocity. It includes a sensor ( 6 ) with a sensor support body ( 13 ) provided with a diaphragm ( 15 ) covering a cavity ( 14 ) formed in the support body ( 13 ). A pressure sensitive element ( 41 ) is mounted on the diaphragm, for recording pressure. Furthermore, a temperature sensitive resistor ( 42 ) is mounted in the vicinity of the pressure sensitive resistor and has a known temperature dependence, for recording temperature. It also includes an electrical circuit ( 43, 44, 45, 46 ) selectively outputting signals from either of the pressure sensitive element and the temperature sensitive resistor.

The present invention relates generally to pressure, temperature andflow measurements, in particular in the medical field, and especially toin situ measurements of the intracoronary pressure, distally of astricture, using a guide wire having a pressure sensor mounted at itsdistal end.

In particular it concerns a combined flow, pressure and temperaturesensor.

BACKGROUND OF THE INVENTION

In order to determine or assess the ability of a specific coronaryvessel to supply blood to the heart muscle, i.e. the myocardium, thereis known a method by which the intracoronary pressure distally of astricture in combination with the proximal pressure is measured. Themethod is a determination of the so called Fractional Flow Reserve (see“Fractional Flow Reserve”, Circulation, Vol. 92, No. 11, Dec. 1, 1995,by Nico H. j. Pijls et al.). Briefly FFR_(myo) is defined as the ratiobetween the pressure distally of a stricture and the pressure proximalof a stricture, i.e. FFR_(myo)=P_(dist)/P_(prox). The distal pressure ismeasured in the vessel using a micro-pressure transducer, and theproximal pressure is the arterial pressure.

A limitation in measuring only the blood pressure and the pressuregradient, alternatively the Fractional Flow Reserve, is that there is nocontrol of the flow in the coronary vessel. As an example, a vesselhaving a significant stricture would not yield any pressure drop if themyocardium is defective and has no ability to receive blood. Thediagnosis will incorrectly show that the coronary vessel is healthy,when instead the conclusion should have been that the myocardium andpossibly the coronary vessel are ill.

A diagnosis method for diagnosing small vessel disease is performed asfollows:

The Fractional Flow Reserve is determined. If the FFR is <0.75 thecoronary vessel should be treated.

If FFR is >0.75 there are two possibilities:

a) either the patient is healthy with respect to the actual coronaryvessel (the most plausible), or

b) there is a low blood flow distally of the distal pressure measurementdue to either an additional stricture or a sickly myocardium.

In order to investigate whether alternative b) is at hand, it isdesirable to obtain knowledge regarding the health status of themyocardium, by measuring Coronary Flow Reserve (CFR), or in thealternative the Coronary Velocity Reserve (CVR). The idea is todetermine by how many times a patient is able to increase his/her bloodflow during work. A healthy patient should be able to increase the bloodflow by 2.5-5 times, depending on the patient's age. Work is simulatedby the addition of a so called vaso dilating pharmaceutical/medicament,e.g. Adenosine, Papaverin or the like. This medicament dilates thecapillaries which increases the blood flow. The same medicament is usedfor determining FFR.

CFV is defined as $\begin{matrix}{{CFV} = \quad {Q_{work}/Q_{rest}}} \\{= \quad {Q_{{during}\quad {vaso}\quad {dilatation}}/Q_{rest}}}\end{matrix}$

(Q is the flow).

This being a ratio and assuming that the cross sectional area isconstant during one velocity measurement, it will suffice to measure thevelocity.

CFR is defined as

CFR=Q_(work)/Q_(rest)=[V_(work)*K]/[V_(rest)*K]=V_(work)/V_(rest)

Since the desired parameter is a flow increase, it will be sufficient toobtain it as a relative quantity

CFR=[K*V_(work)]/[K*V_(rest])

wherein K is a constant.

Researchers have devised methods where the pressure and flow velocity inthe coronary vessel are measured, the results being presented as socalled “pressure-velocity loops” (di Mario). Thereby it becomes possibleto distinguish patients suffering from the so called “small vesseldisease” from others. In patients with “small vessel disease” thepressure gradient, corresponding to a low FFR, and the velocity of flowwill be low, whereas healthy patients will have a low pressure gradient,corresponding to a high FFR, and a high flow.

In some investigations the applicant's system for pressure measurementsin vivo, Pressure Guide™ (Radi Medical Systems) and the flow sensor soldunder the trade name Flowmap™ (Cardiometrics) have been tested.

It is a great drawback to have to introduce two sensors into thecoronary vessel, compared to a situation where both sensors are mountedon a “guide wire”. Thus, it has been suggested to provide a guide wirewith two sensors, but this presents several technical problems with theintegration of two sensors in a thin guide wire.

SUMMARY OF THE INVENTION

The object of the invention is therefor to make available means andmethods for carrying out such combined pressure and flow measurementswith a single unit, thus facilitating investigations of the outlinedtype, and making diagnosing more reliable.

The object outlined above is achieved according to the invention withthe sensor as defined in claim 1, whereby the problems of the prior arthave been overcome. The key is to use the temperature sensitive elementfor obtaining a flow parameter. Thus, there is provided a single sensorhaving the ability to measure both the pressure and to determine thevelocity of flow or the volume flow. A great advantage with such asolution is that only one electrical circuit needs to be provided in aguide wire.

In a preferred embodiment, the sensor is an electrical sensor of apiezoresistive type. However it is contemplated that other pressuresensitive devices may be used, e.g. capacitive devices, or mechanicallyresonating sensors.

In accordance with the invention there is also provided a method ofdetermining pressure, temperature and flow in a coronary vessel, asdefined in claim 20.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter.

However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus not limitative ofthe present invention, and wherein

FIGS. 1a and 1 b show a microphone for recording extremely small eddiesin turbulent gas flows;

FIG. 2 shows a sensor/guide assembly to be used together with theinvention;

FIG. 3 shows a top view of a pressure sensor chip and the electriccircuitry schematically illustrated;

FIG. 4 shows schematically the circuit of a “double” Wheatstone bridgefor use in the invention;

FIG. 5 is an illustration of a Wheatstone bridge used in a secondembodiment of the invention;

FIG. 6 shows temperature profiles obtained in a thermodilution typemeasurement;

FIG. 7 is a schematic illustration showing how transit time is used toobtain the desired parameter.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1a and 1 b there is shown a prior art devicedisclosed in a publication entitled “A Small-Size Microphone forMeasurements in Turbulent Gas Flows” in Sensors and Actuators A, 1994.It comprises a microphone for recording extremely small eddies inturbulent gas flows. It is based on piezoresistive techniques fortransducing pressure fluctuations into electrical signals.

The microphone comprises a silicon substrate 100, and a cavity 102 insaid substrate. A diaphragm of polysilicon 104 covers the cavity 102. Onthe diaphragm a polysilicon piezoresistor 106 is attached. Etch holes108 and etch channels 110 are provided for manufacturing purposes. Ventchannels 112 are also provided. On the substrate 100 there are metalconductors 114 and bond pads 116 for connecting cabling to externaldevices.

Now turning to FIG. 2 there is shown a sensor/guide device comprising asolid wire 1 which is machined by so called centering grinding, andinserted into a proximal tube portion 2. The wire 1 forms the distalportion of the guide, and extends beyond the distal end of the proximaltube portion 2 where said tube is connected to or integrally formed witha spiral portion 3. On the distal end of the wire 1 there is mounted apressure sensor 6. Between the wire 1 and the spiral portion 3,electrical leads 4 from the electronic circuitry run parallel with saidwire 1. The sensor 6 is protected by a short section of a tube 7 havingan aperture 8 through which surrounding media act on the pressuresensor. At the very distal end of the entire device there is a radioopaque coil 9, e.g. made of Pt, and used for location purposes, and asafety wire 10 for securing the distal part of the spiral 9.

To minimize the number of electrical leads, the wire or tube may be usedas one of the electrical leads.

The proximal tubing 2 and the spiral 3 may be coupled such as to beutilized as an electrical shield, in which case it of course cannot beused as an electrical lead.

Now embodiments of the pressure sensor will be described with referenceto FIGS. 3-4.

The sensor is based on the small size silicon microphone mentionedabove, which is designed for detecting extremely small eddies inturbulent gas flows. It has been fully described for that application insaid publication “Sensors and Actuators A”, 1994 (incorporated herein inits entirety by reference). However, it has been modified in accordancewith the present invention in the way described below. In order tofurther miniaturize the external dimensions of the microphone to meetthe requirements of the invention, the external dimensions foraccommodating the lead pattern on the sensor should be no more than 0.18mm×1.3 mm×0.18 mm, preferably no more than 0.14 mm×1.3 mm×0.1 mm.

An unexpected advantage of miniaturizing is that the thermal mass, andthereby the thermal time constant, is low, i.e. the entire chipincluding its resistors heats up and cools down very quickly. In fact itis thereby possible to monitor dynamic changes in the domain 1 Hz andfaster. For the purpose of studying flow in blood vessels, the variationof flow velocity or volume flow during a heart cycle is easily detected,and therefor anomalies in the blood flow may be detected.

The sensor (see FIG. 3) comprises a sensor support body in the form of asilicon chip 13 in which there is a cavity 14 made e.g. by etching.Across the cavity there is formed a polysilicon diaphragm 15 having athickness of e.g. 0.4-1.5 μm or possibly up to 5 μm, and a side lengthof 100 μm. Within the cavity a vacuum of less than 1000 Pa, preferablyless than 30 Pa prevails. In contact with said diaphragm there ismounted a piezoresistive element 41. A pressure acting on the diaphragm15 will cause a deflection thereof and of the piezoresistive element 41,which yields a signal that may be detected.

In order to attach the cabling 4 to the chip, bond pads 19 are required.These bond pads must have a certain dimension (e.g. 100×75 μm), and mustbe spaced apart a certain distance, respect distance approximately 125μm. Since the dimensional adaptation entails narrowing the chip, theconsequence is that in order to be able to meet the mentionedrequirements, the bond pads have to be located in a row, one after theother, as shown in FIG. 3.

It is also preferred for temperature compensation purposes to have areference resistor 42 mounted on the sensor. This reference resistor 42may be located on different points on the sensor chip.

In one embodiment it is placed on the diaphragm 15. This is preferredsince identical environments to both the active, piezoresistive element41 and the reference resistor 42 will be provided. Thereby the activeelement, i.e. the piezoresistive element 41, is mounted such that itwill be affected by a longitudinal tension 41 when it is subjected to apressure. The reference resistor 42 is preferably mountedperpendicularly with respect to the active element 41 and along theborder of the diaphragm 15, i.e. at the periphery of the cavity 14present underneath the diaphragm 15.

However, it is possible to locate the reference resistor on the siliconsubstrate 13 adjacent the diaphragm. This is an advantage since thereference resistance thereby will be pressure independent.

Another possibility is to locate the reference resistor on a “dummy”diaphragm adjacent the real diaphragm 15, in order to provide the samemechanical and thermal environment for the active element 41 and thereference resistor 42.

With reference to FIG. 4, an embodiment of the electrical circuit andits operation and function will now be described.

As schematically is shown in FIG. 4, one embodiment of the sensorcircuit comprises six resistors 41 . . . 46, two of which 41, 42 aremounted on the diaphragm, as previously mentioned (resistor 41corresponds to resistor 41 in FIG. 3, and resistor 42 corresponds toresistor 42 in FIG. 3). Resistor 41 is a piezoresistive element, andresistor 42 is only temperature sensitive. The remaining resistors 43,44, 45, 46 are located externally of the entire sensor/guide assembly,and do not form part of the sensor element.

In this embodiment the resistors are coupled as a “double” Wheatstonebridge, i.e. with resistors 42, 43, 44, 46 forming one bridge (fortemperature compensation and flow calculation), resistors 41, 42, 42, 46forming the second bridge for pressure measurement. Thus, resistors 45and 46 are shared by the bridges. Thereby it is possible to measure thetemperature (across B-C) and pressure (across A-C) independently of eachother. From the measured temperature values the flow velocity or volumeflow may be calculated.

In another embodiment there are four resistors (51, 52, 53, 54)connected as shown in FIG. 5, i.e. as a simple “single” Wheatstonebridge. If at least one of the four resistors, say 51, has a temperaturecoefficient ≠0, then temperature changes may be measured as follows:

If the voltage V applied is maintained constant, the current I throughthe circuit may be measured and is a measure of the temperature, sincethe total impedance (resistance) of the circuit will change withtemperature.

Alternatively the current I may be maintained constant, and in this casethe voltage over the bridge will be temperature dependent.

By means of the shown circuit, the CFR can be determined by registeringthe temperature drop due to a passing liquid having a lower temperaturethan the body temperature, as will be discussed in detail below.

For the flow determination the principle of so called hot-wire andhot-film anemometers may be employed (reference is made to “MeasurementSystems”, 3rd edition, pp 506-, by Doebelin, 1983), in which case a flowvelocity may be obtained.

Alternatively the principle of thermo-dilution may be employed in whichcase the volume flow may be obtained.

Both principles will be discussed below beginning with hot-wireanemometers.

Hot-wire anemometers commonly are made in two basic forms: the constantcurrent type and the constant temperature type. Both utilize the samephysical principle but in different ways. In the constant current type,a fine resistance wire carrying a fixed current is exposed to the fluidflowing at a certain velocity. The wire attains an equilibriumtemperature when the i²R heat is essentially constant; thus the wiretemperature must adjust itself to change the convective loss untilequilibrium is reached. Since the convection film coefficient is afunction of flow velocity, the equilibrium wire temperature is a measureof velocity. The wire temperature can be measured in terms of itselectrical resistance. In the constant temperature form, the currentthrough the wire is adjusted to keep the wire temperature (as measuredby its resistance) constant. The current required to do this thenbecomes a measure of flow velocity.

For equilibrium conditions we can write an energy balance for a hot wireas

I²R_(w)=hA(Tw−T_(f))

where

I=wire current

T_(w)=wire temperature

T_(f)=temperature of flowing fluid

h=film coefficient of heat transfer

A=heat transfer area

R_(w)=wire resistance

h is mainly a function of flow velocity for a given fluid density.

It can be written generally on the form

h=C₀+C₁{square root over (V)}

where V is the flow velocity, and C₀ and C₁, are constants. For a moredetailed account of the theory for hot-wire anemometers reference ismade to the cited publication.

In pressure measurement mode the resistors in the circuit (FIG. 4) aresupplied with 1-10 V (AC or DC), and the potential difference between Aand B is registered as a signal representing the pressure. Unless theresistors 41 and 42 are identical in terms of their temperaturedependence, this potential difference will be temperature dependent,i.e. one has to know a quantity representative of the temperature atwhich the measurement takes place in order to obtain a correct pressurevalue, and therefore the bridge has to be calibrated. This is achievedby recording the potential difference between A and B (see FIG. 4) as afunction of the potential difference between A and C at differenttemperatures, e.g. in a controlled temperature oven or in a water bath.Thus, an “off set” vs temperature dependence curve is obtained, that isused to compensate the pressure signal (A-B) for a given temperature.Namely, at a given temperature it is known from the calibration curvehow much should be subtracted from or added to the actual registeredsignal in order to obtain a correct pressure. It would be advantageousif resistors 41 and 42 have identical or at least a very similartemperature dependence. This is in fact also the case, since they aremade in practice at the same time during manufacture of the chip itself.Thus, material composition and properties are in practice identical.Nevertheless the above outlined compensation is necessary in most cases.

The actual compensation process is built into the software of theelectronic system, and implementation thereof requires only ordinaryskill.

The inventors have now realized that it is possible to make use of thetemperature dependent resistor in a pressure bridge as described above,for flow measurements, using the principle of the hot-wire anemometer.

Thus, the temperature sensitive resistor 42 (FIG. 4) having a knowntemperature behavior as a function of the current supplied to it, is fedwith a current that in a static situation (i.e. no flowing fluidsurrounding it) would yield a certain temperature, as reflected in itsresistance. If there is a difference in the measured resistance comparedto what would have been expected in the static situation (i.e. no flow),it can be concluded that a cooling of the resistor is taking place, andthus that there is a flow of fluid. The measurement is made over B-C inthe figure. On the basis of this information, the theory indicated abovefor anemometers may be applied, and a flow velocity calculated.

The CFR value may be obtained in the following way using the anemometerprinciple:

1. place a sensor distally of a suspected stricture

2. register the flow parameter (“flow velocity”) in a rest condition,V_(rest)*K (K is a constant)

3. inject a medicament (e.g. Adenosin, Papaverin) for vaso dilatation

4. register the flow parameter (“velocity”) in a work condition,V_(max)*K (K is a constant)

5. calculate CFR=V_(max)/V_(rest)

During the same procedure the FFR (Fractional Flow Reserve) may also beobtained by measuring the distal and proximal pressures and calculatingFFR=P_(dist)/P_(prox).

Now the embodiment utilizing the principle of thermo-dilution will bedescribed.

The principle of thermo-dilution involves injecting a known amount ofcooled liquid, e.g. physiological saline in a blood vessel. Afterinjection the temperature is continuously recorded with a temperaturesensor attached to the tip of a guide wire that is inserted in thevessel. A temperature change due to the cold liquid passing themeasurement site, i.e. the location of the sensor, will be a function ofthe flow (see FIG. 5).

There are various methods of evaluating the temperature signal fordiagnostic purposes. Either one may attempt to calculate the volumeflow, or one may use a relative measure, where the flow in a “restcondition” is compared with a “work condition”, induced by medicaments.

The latter is the simpler way, and may be carried out by measuring thewidth at half height of the temperature change profile in the twosituations indicated, and forming a ratio between these quantities (seeFIG. 6).

Another way of obtaining a ratio would be to measure the transit timefrom injection and until the cold liquid passes the sensor, in restcondition and in work condition respectively. The relevant points ofmeasurement are shown in FIG. 7.

The former method, i.e. the utilization of the volume flow parameter assuch, requires integration of the temperature profile over time (seeFIG. 6) in accordance with the equations given below $\begin{matrix}{Q_{rest} = {{V/{\int_{t_{0}}^{t_{1}}{\left( {T_{r,{m/}}T_{r,1}} \right){t}}}} \propto {V/{\int_{t_{0}}^{t_{1}}\left( {T_{r,0} - {T_{{r,m})}{t}}} \right.}}}} & (1) \\{Q_{work} = {{V/{\int_{t_{0}}^{t_{1}}{\left( {T_{w,{m/}}T_{w,1}} \right){t}}}} \propto {V/{\int_{t_{0}}^{t_{1}}\left( {T_{w,0} - {T_{{w,m})}{t}}} \right.}}}} & (1)\end{matrix}$

wherein

V is the volume of injected liquid

T_(r,m) is the measured temperature at rest condition

T_(r,l) is the temperature of injected liquid at rest condition

T_(o) is the temperature of the blood, i.e. 37° C.

T_(w,m) is the measured temperature at work condition

T_(w,l) is the temperature of injected liquid at work condition

Q is the volume flow

These quantities may then be used directly for assessment of thecondition of the coronary vessels and the myocardium of the patient, orthey may be ratioed as previously to obtain a CFR, i.e.CFR=Q_(work)/Q_(rest).

A method of diagnosing small vessel disease, using the device of theinvention comprises performing measurements at a site in a vesseldistally of a suspected stricture. Thus, a pressure sensitive elementand a resistor on a sensor element is provided at a measurement site, byinserting through a catheter. The pressure sensitive element and saidresistor are part of an electric circuit yielding a pressure indicativeoutput and a temperature indicative output, and have known temperaturedependencies. The resistor is used as a reference for the pressuresensitive element. At the site the sensor element will be subjected toflowing fluid, i.e. blood, and the temperature of said fluid ismonitored by continuously recording said temperature indicative outputfrom said electric circuit. Then said resistor is subjected to a changedthermal environment. The change in said temperature indicative outputresulting from said changed thermal environment is registered. Thischange in temperature indicative output is used to calculate a flowparameter (Q_(rest)). A vaso dilating drug is injected in said vessel tosimulate a work condition, and the distal pressure (P_(work,dist)) andtemperature of said fluid is monitored by continuously recording saidpressure indicative output and said temperature indicative output fromsaid electric circuit. Again the resistor is exposed to a changedthermal environment, and the change in said temperature indicativeoutput resulting from said changed thermal environment is registered. Aflow parameter (Q_(work)) is calculated from said change in saidtemperature indicative output. The proximal pressure (P_(prox,work)) isdetermined, and CFR=Q_(work)/Q_(rest) andFFR=P_(dist,work)/P_(prox,work) are calculated. Finally the calculatedCFR and FFR are compared with corresponding quantities representative ofa healthy patient.

The invention being thus described, it will be clear that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be clear to one skilled in the art are intendedto be included within the scope of the following claims.

In particular it may find utility in other areas of the medical field,wherever it is desired to measure pressure, temperature and flow withone single device. It could also be used in non-medical fields.

What is claimed is:
 1. A device for determining pressure, temperatureand a flow parameter of a flowing fluid, comprising i) a sensor supportbody (13) mounted for insertion into a vessel of a living body andhaving a diaphragm (15) covering a cavity (14) formed in said supportbody (13); ii) a pressure sensitive element (41), having a knowntemperature dependence, mounted on said diaphragm (15), and providing anoutput indicative of a pressure; iii) a temperature sensitive resistor(42) mounted in the vicinity of said pressure sensitive element (41) andhaving a known temperature dependence, providing an output signalindicative of a temperature, said resistor (42) also functioning as atemperature reference means for said pressure sensitive element (41);and iv) an electrical circuit (43, 44, 45, 46), connected to thepressure sensitive element and to the temperature sensitive resistor,for calculation of a flow parameter of said flowing fluid on the basisof the temperature signals.
 2. The device of claim 1, wherein saidelectrical circuit comprises a double Wheatstone bridge, including afirst (42, 43, 44, 46) and a second (41, 42, 45, 46) bridge, said twobridges having two resistors in common, whereby the first bridgecomprises said temperature sensitive resistor (42), and the secondbridge comprises the pressure sensitive element (41) and saidtemperature sensitive resistor (42).
 3. The device as claimed in claim1, comprising a Wheaststone bridge, wherein the output from the bridgeis indicative of pressure, and the total impedance of the bridge isindicative of temperature.
 4. The device as claimed in claim 1, whereinboth the pressure sensitive element (41) and the temperature sensitiveresistor (42) are located on said diaphragm (15).
 5. The device asclaimed in claim 1, wherein only the pressure sensitive element (41) islocated on said diaphragm (15) and the temperature sensitive resistor(42) is located on said sensor support body (13).
 6. The device asclaimed in claim 1, wherein the temperature sensitive resistor (42) ismounted on a dummy diaphragm, having essentially the same properties asthe diaphragm (15) on which the pressure sensitive element is located.7. The device as claimed in claim 1, attached at the distal end of aguide wire having a proximal and a distal end.
 8. The device as claimedin claim 1, further comprising means for temperature compensation in thepressure measurement mode, such that the recorded potential representinga pressure is modified by adding or subtracting from said recordedpotential, a known off set potential value depending on temperature. 9.The device as claimed in claim 1, wherein the pressure sensitive element(41) is of a piezoresistive type.
 10. The device as claimed in claim 1,wherein the pressure sensitive element (41) is of a capacitive type. 11.The device as claimed in claim 1, wherein the pressure sensitive element(41) is a mechanically resonant sensor.
 12. The device of claim 1,further including a selective power supply power connected to saidtemperature sensitive resistor to heat the temperature sensitiveresistor to a predetermined temperature, and a temperature deviationdeterminator.
 13. A guide wire and sensor assembly for determiningpressure, temperature and a flow parameter of a fluid flowing in aliving body, comprising i) a guide wire (2) having a distal and aproximal end; ii) a sensor element (6) provided at the distal end ofsaid guide wire, said sensor element comprising a) a sensor support body(13) provided with a diaphragm (15) covering a cavity (14) formed insaid support body; b) a pressure sensitive element (41) having a knowntemperature dependence and mounted on said diaphragm (15), recordingpressure; and c) a temperature sensitive resistor (42) mounted in thevicinity of said pressure sensitive element (41) and having a knowntemperature dependence, recording temperature, said resistor (42) alsofunctioning as a temperature reference for the pressure sensitiveelement (41) and to provide temperature signals for calculation of aflow parameter; and iii) an electrical circuit comprising a doubleWheatstone bridge, including a first (42, 43, 44, 46) and a second (41,42, 45, 46) bridge, said two bridges together comprising six resistiveelements, said double Wheatstone bridge having two resistors in common,whereby the second bridge includes said pressure sensitive element (41)and said temperature sensitive resistor (42), and the first bridgeincludes the temperature sensitive resistor (42).
 14. A guide wire andsensor assembly for determining pressure, temperature and a flowparameter of a fluid flowing in a vessel, comprising i) a guide wire (2)having a distal and a proximal end; ii) a sensor element (6) provided atthe distal end of said guide wire, said sensor element comprising a) asensor support body (13) provided with a diaphragm (15) covering acavity formed in said support body (13); b) a pressure sensitive element(41) having a known temperature dependence and mounted on said diaphragm(15), recording pressure; and c) a temperature sensitive resistor (42)mounted in the vicinity of said pressure sensitive element and having aknown temperature dependence, recording temperature, said resistor alsofunctioning as a temperature reference for the pressure sensitiveelement and to provide temperature signals for calculation of a flowparameter; and iii) an electrical circuit selectively recording outputsignals from either of said pressure sensitive element and saidresistor, said circuit comprising a Wheatstone bridge (51, 52, 53, 54),wherein the output from the bridge is indicative of pressure, and thetotal impedance of the bridge is indicative of temperature.
 15. Themethod of claim 14, wherein the flow of fluid comprises a primary and asecondary fluid, divided up in two sequential flows having differenttemperatures, the primary flow comprising blood, and the secondary flowbeing a fluid of substantially lower temperature than the blood.
 16. Themethod of claim 15, wherein a temperature profile is obtained by thecontinuous measurement of the temperature, and wherein the width at halfheight of said temperature profile is used as flow parameter.
 17. Themethod of claim 15, wherein the time for the cold fluid to pass thesensor is used as said flow parameter.
 18. The method of claim 15,wherein the time of transit of the cold fluid from the time of injectionuntil it reaches said sensor is used as said flow parameter.
 19. Amethod of determining pressure, temperature and a flow parameter offluid flowing in vessels, comprising the following steps: a) providing apressure sensitive element and a resistor on a sensor element at ameasurement site in a vessel of a living body, said pressure sensitiveelement and said resistor being part of an electric circuit yielding apressure indicative output and a temperature indicative output, saidpressure sensitive element and said resistor having known temperaturedependencies, whereby the resistor is used as a reference for thepressure sensitive element; b) subjecting said sensor element to flowingfluid and monitoring the pressure and temperature of said fluid bycontinuously recording said pressure indicative output and saidtemperature indicative output from said electric circuit; c) subjectingsaid resistor to a changed thermal environment; d) registering thechange in said temperature indicative output resulting from said changedthermal environment; and e) calculating a flow parameter from saidchange in said temperature indicative output.
 20. The method of claim19, including achieving said changed thermal environment by said fluidcausing a temperature drop in said resistor, said flow parameter being aquantity proportional to the volume flow.
 21. The method of claim 20,wherein the volume flow is calculated by integrating the temperatureover time using the equation $\begin{matrix}{Q = {{V/{\int_{t_{0}}^{t_{1}}{\left( {T_{m/}T_{1}} \right){t}}}} \propto {V/{\int_{t_{0}}^{t_{1}}\left( {T_{0} - {T_{m)}{t}}} \right.}}}} & (1)\end{matrix}$

wherein V is the volume of injected liquid T_(m) is the measuredtemperature T_(l) is the temperature of injected liquid T_(o) is thetemperature of the blood, i.e. 37° C. Q is the volume flow t_(o) is thepoint in time where a temperature change is detected t₁ is the point intime where the temperature is regarded as having reached normaltemperature.
 22. The method of claim 19, wherein said changed thermalenvironment is achieved by said resistor being heated by passing acurrent through it, whereby said fluid cools said resistor such that theactual temperature of said resistor is lower than the expectedtemperature, that would have been obtained, had said resistor not beensubjected to said flowing fluid, and calculating a flow parameter fromsaid deviation from said expected temperature value, said flow parameterbeing a quantity proportional to the flow velocity.
 23. The method ofclaim 22, wherein a flow velocity is calculated using the equationV=(h−C₀)²/C₁ wherein h=I²R_(w)/A(T_(w)−T_(f)) wherein I=wire currentR_(w)=wire resistance T_(w)=wire temperature T_(f)=temperature of aflowing fluid h=film coefficient of heat transfer A=heat transfer areaV=flow velocity.
 24. A method of diagnosing small vessel disease,comprising performing a measurement at a site in a vessel distally of asuspected stricture according to the following steps: a) providing apressure sensitive element and a resistor on a sensor element at ameasurement site, said pressure sensitive element and said resistorbeing part of an electric circuit yielding a pressure indicative outputand a temperature indicative output, said pressure sensitive element andsaid resistor having known temperature dependencies, whereby theresistor is used as a reference for the pressure sensitive element; b)subjecting said sensor element to flowing fluid and monitoring thepressure and temperature of said fluid by continuously recording saidpressure indicative output and said temperature indicative output fromsaid electric circuit; c) subjecting said resistor to a changed thermalenvironment; d) registering the change in said temperature indicativeoutput resulting from said changed thermal environment; and e)calculating a flow parameter from said change in said temperatureindicative output; and f) comparing the calculated flow parameter andthe measured pressure with corresponding quantities representative of ahealthy patient.
 25. The method of claim 24, wherein said comparing stepf) comprises comparing a flow parameter in a rest condition with a flowparameter in a work condition, and the pressure distally of a stenosiswith the proximal pressure in a work condition.
 26. A method ofdiagnosing small vessel disease, comprising performing measurements at asite in a vessel distally of a suspected stricture according to thefollowing steps: a) providing a pressure sensitive element and aresistor on a sensor element at a measurement site, said pressuresensitive element and said resistor being part of an electric circuityielding a pressure indicative output and a temperature indicativeoutput, said pressure sensitive element and said resistor having knowntemperature dependencies, whereby the resistor is used as a referencefor the pressure sensitive element; b) subjecting said sensor element toflowing fluid and monitoring the temperature of said fluid bycontinuously recording said temperature indicative output from saidelectric circuit; c) subjecting said resistor to a changed thermalenvironment; d) registering the change in said temperature indicativeoutput resulting from said changed thermal environment; e) calculating aflow parameter (Q_(rest)) from said change in said temperatureindicative output; f) injecting a vaso dilating drug in said vessel tosimulate a work condition; g) monitoring the pressure (P_(work,dist))and temperature of said fluid by continuously recording said pressureindicative output and said temperature indicative output from saidelectric circuit; h) subjecting said resistor to a changed thermalenvironment; i) registering the change in said temperature indicativeoutput resulting from said changed thermal environment; j) calculating aflow parameter (Q_(work)) from said change in said temperatureindicative output; k) determining the proximal pressure (P_(prox,work));l) calculating CFR=Q_(work)/Q_(rest) and FFR=P_(dist,work)/P_(prox,work)j) comparing the calculated CFR and FFR with corresponding quantitiesrepresentative of a healthy patient.
 27. A device for biologicalpressure and temperature measurements, comprising: a guide wire; apressure sensor mounted on the guide wire; a temperature sensor mountedon the guide wire in the vicinity of the pressure sensor; and anelectronic circuit to generate and output an indication of temperaturebased on signals from the temperature sensor.
 28. A device as set forthin claim 27, further comprising a sensor support body and wherein thepressure sensor and the temperature sensor are both mounted to saidsensor support body.
 29. A device as set forth in claim 27, furthercomprising a chip and wherein the pressure sensor and the temperaturesensor are both mounted to said chip.
 30. A device as set forth in claim27, further comprising a silicon chip and wherein the pressure sensorand the temperature sensor are both mounted to said silicon chip.
 31. Adevice as set forth in claim 27, further comprising a substrate andwherein the pressure sensor and the temperature sensor are both mountedto said substrate.
 32. A device as set forth in claim 27, wherein thepressure sensor senses pressure at the same time the temperature sensorsenses temperature.
 33. A device as set forth in claim 27, wherein thepressure sensor includes a diaphragm.
 34. A device as set forth in claim33, wherein the temperature sensor is located off of the diaphragm. 35.A device as set forth in claim 27, wherein the electronic circuit alsogenerates a profile of temperature versus time.
 36. A device as setforth in claim 35, wherein the electronic circuit also generates flowinformation based on said profile.